Offering a no deposit bonus is a completely legal marketing strategy for online casinos. There are no state or federal US laws forbidding no deposit bonuses. Having said that, it is the responsibility of the player to ensure the bonus has fair and reasonable terms and stipulations.
The legality of online casino gaming in the United States differs from one state to another, basically each state gets to make its own laws pertaining to online gambling and gambling in general. Online casinos are permitted in states like Pennsylvania, Delaware, and New Jersey. In a few other states, like West Virginia for example, the legalisation of online casino gaming is in the pipeline and should manifest soon.
However, internet casinos are prohibited in all other states, and firms situated in the United States who provide such services to US customers can be prosecuted by law. Offshore casinos, however, lie outside the jurisdiction of the US and hence aren’t subjected to the laws of the state. Therefore, US based players can play real money games at offshore casinos without any risk of being in violation of the laws. Players should, however, only play at reliable and reputed casinos when playing at offshore casinos.